1. Field of the Invention
This invention relates to suspension units of the type for supporting a tilt cab of a vehicle on the frame of the vehicle, and in particular to such a unit for supporting the rear portion of a cab in which the cab is pivotally mounted to the frame near the front.
2. Discussion of the Prior Art
In tilt cab trucks, such as over-the-road and yard trucks, the cab is typically mounted at its front to the frame using anti-vibration bushings and the rear of the cab is supported on the frame by a suspension unit which includes one or more springs, typically air springs controlled by a height control valve. The unit is usually damped by one or more shock absorbers. Such suspension units are disclosed, for example, in U.S. Pat. Nos. 4,989,684; 5,109,939; 5,209,316 and 5,253,853.
U.S. Pat. No. 5,579,860 discloses a three-point cab mounting system which is especially adapted for a yard truck of the type which has the center of gravity of the cab offset to one side of the center of the truck frame. The front of the cab is pivotally mounted to the frame with anti-vibration bushings and a single cab suspension unit, which is bolted to the outside of one of the side rails of the frame of the vehicle, supports the rear of the cab. The present invention is directed to a vehicle cab suspension unit which is specially, although not exclusively, adapted to be used in such a three-point cab mounting system.
It has been common in prior cab suspension units for the spring of the suspension unit to support the entire weight of the cab, including the operator and any items placed in the cab. Thus, for sufficient capacity, it has been necessary to use two air springs, or if only one was used, that it be capable of bearing the entire load of the cab. Therefore, it is an object of the invention to provide a suspension unit which can utilize lower capacity springs.
In addition, it is advantageous in vehicle suspension units to orient the air springs at an angle so as to lower their natural frequency. In a three-point system, in which the suspension unit is mounted to the outside of a frame rail and the connection to the cab is made outside of the frame rail, how an air spring could be mounted at an angle in a suspension unit has not been obvious. For example, in the first embodiment of the suspension unit disclosed in U.S. Pat. No. 5,579,860, the load bearing element, which is a marshmallow isolator, has its axis oriented vertically. In the embodiments of FIGS. 7A-C, a suspension unit of a type which is commercially available from the assignee of the present patent is disclosed as an alternative, although this unit employs two angled air springs which together support the full weight of the rear of the cab. It is therefore a relatively large suspension unit and is not particularly adapted for a side mount application, with limited space available for the suspension unit and so as to impart certain stabilizing forces to the cab.